1. Field of the Invention
This invention relates to devices for the interruption of DC power transmission lines. In particular, the invention provides for switching DC power without substantial arcing using solid state rectifiers such as thyristors in high voltage, high current transmission systems, typically 100 kV DC and 1 kA DC and above.
2. Description of the Prior Art
In high voltage DC (HVDC) transmission systems heavy duty power circuit breakers are used to open circuit or deenergize DC transmission lines. These breakers perform a dual function--opening the line for normal operating conditions such as inspection and maintenance and for correcting fault conditions such as a short circuit. During either condition, when the circuit breaker is opened while current is flowing in the transmission line, an electric arc is generated between the contacts of the breaker as they separate. This electric arc continues conducting current into the transmission line and together with other system parameters causes voltage transients to be generated. The magnitude of these transients can be several times greater than the normal system voltage and can cause extensive damage to equipment connected to the transmission line as well as to the transmission line itself. Therefore, it would be desirous to eliminate or reduce the magnitude of these transients during circuit interruption. Various means have been developed to minimize the magnitude of the arc and the subsequent voltage transients. These means include puffer devices to blow out the arc, use of SF.sub.6 insulating gas and stored energy devices to rapidly separate the breaker contacts. However, as the voltage and current ratings of the transmission lines increase, these interruption devices must also be increasingly ruggedized in order to circumvent the arcing and voltage transient problem.
The electric arc which is generated during the opening of a breaker is a complex function of the transmission voltage, current, system inductance and the amount of time required to open the device and extinguish the arc. Present DC breakers have operating times in the range of about 100 milliseconds. In comparing this operating time to a 60 cycle AC system, approximately 6 cycles would be required for the operation of the breaker. By decreasing the operating time for circuit interruption, the magnitude of the transients can be reduced. In addition, if the transmission line current were reduced prior to operation of the circuit breaker, a reduction in the resultant arcing and transients would also occur. Thus, it would be advantageous to have a device which can offer rapid operating times as well as allowing circuit interruption with substantially reduced arcing and transients.